git-send-email: skip RFC2047 quoting for ASCII subjects

The git-send-email always use RFC2047 subject quoting for
files with "broken" encoding - non-ASCII files without
Content-Transfer-Encoding, even for ASCII subjects. This is
harmless but unnecessarily ugly for people reading the raw
headers. This patch skips rfc2047 quoting when the subject
does not need it.

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: Jeff King <peff@peff.net>

git-send-email.perl

@@ -1327,7 +1327,8 @@ foreach my $t (@files) {
 		$body_encoding = $auto_8bit_encoding;
 	}
 
-	if ($broken_encoding{$t} && !is_rfc2047_quoted($subject)) {
+	if ($broken_encoding{$t} && !is_rfc2047_quoted($subject) &&
+			($subject =~ /[^[:ascii:]]/)) {
 		$subject = quote_rfc2047($subject, $auto_8bit_encoding);
 	}
 

t/t9001-send-email.sh

@@ -1142,6 +1142,23 @@ Dieser deutsche Text enthält einen Umlaut!
 EOF
 '
 
+test_expect_success $PREREQ 'setup expect' '
+cat >expected <<EOF
+Subject: subject goes here
+EOF
+'
+
+test_expect_success $PREREQ 'ASCII subject is not RFC2047 quoted' '
+	clean_fake_sendmail &&
+	echo bogus |
+	git send-email --from=author@example.com --to=nobody@example.com \
+			--smtp-server="$(pwd)/fake.sendmail" \
+			--8bit-encoding=UTF-8 \
+			email-using-8bit >stdout &&
+	grep "Subject" msgtxt1 >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success $PREREQ 'setup expect' '
 cat >content-type-decl <<EOF
 MIME-Version: 1.0